The present invention pertains to biologically active, polypeptide fragments of mammalian bactericidal/permeability-increasing proteins and methods for making and using said fragments.
Bactericidal/permeability-increasing protein (BPI) is a 50 to 60 Kd protein, isolated from the granules of mammalian polymorphonuclear leukocytes (PMN) which are blood cells that are essential in the defense against invading microorganisms in mammals. BPI occurs only in cells of the myeloid series of blood cells, is produced at the promyelocytic/myelocytic stage of differentiation and is located in the primary granules in these cells.
BPI is a potent bactericidal agent active against a broad range of gram-negative bacterial species. It exhibits a high degree of specificity in its cytotoxic effect, i.e. 10-40 nM (0.5-2.0 micrograms), producing greater than 90% killing of 10.sup.7 sensitive bacteria whereas 100-fold higher concentrations of BPI are non-toxic for other microorganisms and eukaryotic cells. All available evidence suggests that in the intact PMN and in crude leukocyte fractions, BPI is the principal oxygen-independent agent present which is active against BPI-sensitive bacteria.
BPI isolated from both human and rabbit PMN has been purified to homogeneity. The molecular weight of human BPI is approximately 58,000 Daltons (58 kDa) and that of rabbit BPI is approximately 50 kDa. The amino acid composition of these two proteins is closely similar as is the amino acid sequence of their first 15 NH.sub.2 -terminal amino acid residues. Both proteins are highly basic, having an isoelectric point greater than 9.6.
The biological effects of BPI require attachment of the protein to the surface of the susceptible gram-negative bacteria. Initial binding of BPI to target cells involves electrostatic interactions between the basic protein and the negatively charged sites on the lipopolysaccharides (LPS) on the bacterial outer membrane and leads to an activation of bacterial enzymes that degrade phospholipids and peptidoglycans. The final stage of action is the actual killing of the bacteria by an as yet unknown mechanism. The closely similar amino acid composition and nearly identical bactericidal and membrane-perturbing properties of BPI purified from human and rabbit PMN suggest that this protein has been highly conserved during evolution and is an important member of the anti-bacterial arsenal of the mammalian PMN.
Due to its potent bactericidal action against gram-negative bacteria and lack of cytotoxicity towards other microorganisms and eukaryotic cells, it is envisioned that BPI may be employed as a chemotherapeutic agent and/or as a model for the design of new antibiotic agents. However, due to its large molecular weight (58 kDa for the human holoprotein), both sequencing and determination of the structural organization of BPI have been hampered (hereinafter the entire BPI molecule is referred to as the holoprotein). The possibility has been raised that, as in the case with other cytotoxic proteins, BPI has a structural organization where the different functions, namely binding, envelope-altering and killing reside in different domains within the BPI molecule. Although BPI fragments, obtained by digestion of the holoproteins with the proteolytic enzyme elastase, has been disclosed (Weiss, J. et al., Clin. Res 34: 537A, 1986), the fragments tested remained associated under the non-denaturing conditions employed. No biological activity was ascribed to any isolated fragments. Moreover, antibodies directed against the holoprotein did not recognize these fragments under denaturing conditions when analyzed using the well-known Western blotting procedure.
Therefore, in light of the above, there is a need in the art for biologically active peptide fragments of BPI for use as bactericidal/permeability increasing agents as well as therapeutic agents. Such fragments are also needed to provide sequence information on BPI to direct the design of future generations of antimicrobial agents specific for gram-negative bacteria and to be used as probes into the molecular organization of the holoproteins.